The present invention relates to a circuit for regulating a voltage controlled oscillator, in particular for controlling a unit for reading and writing data on a magnetic carrier, in which the oscillator is supplied at its input with a bias voltage and a phase displacement voltage (offset voltage).
Since the tolerances in the various components mean that the two control voltages may generate an output frequency which is slightly different from that intended for the recording and reading of the magnetic carrier, for example a floppy disk, reading of the disk may be compromised if the disk has been recorded on a different unit from the reading unit. It is therefore necessary precisely to calibrate the multivibrator in such a way as precisely to generate the desired frequency.
In a known multivibrator the control voltage is generated by an operational amplifier while the frequency generated thereby depends on the above-mentioned voltage and the capacitance of a capacitor. The calibration operation is effected by varying the capacitance of the capacitor which must therefore be of variable type, after mounting on the board. That regulating circuit on the one hand is liable to go out of tune in the course of time while on the other hand it is of a complicated and expensive construction and does not make it possible to produce an integrated circuit which also includes the element to be calibrated.